Rotational transitions from seven low-lying vibrational states of HCCN and five low-lying vibrational states of DCCN have been detected in the frequency range from 100 to 400 GHz with (+/-1) a sensitive millimeter-wave spectrometer. The CCH bending states nu(5)(+/-1), 2 nu(5)(+/-2), and 3 nu(5)(+/-3), and the CCN bending state nu(4)(+/-1) have been assigned. In addition, transitions from three vibrational states in HCCN and one in DCCN with zero orbital angular momentum (l) were also detected. These states in all likelihood originate from the three lowest l=0 excited states, i.e., (nu(4)+nu(5)>(-0) (nu(4) + nu(5))(+0), and 2 nu(5)(0). Analysis of the high-accuracy millimeter-wave frequency data establishes that HCCN is not a near-rigid bent molecule and intensity measurements confirm that the CCH bending states are much lower in energy than in typical well-behaved linear molecules. The low barrier to linearity in HCCN and DCCN of similar to 235 cm(-1), estimated from intensity measurements and the ab initio calculations of Malmquist et al. [Theor. Chim. Acta 73, 155 (1988)], confirms that HCCN is quasilinear.